A predictable working distance between the condenser, specimen, and objective lens is required for microscopy. Microscope slides are made with a specific thickness to place the specimen at the proper working distance between these components. When a slide is coated with mounting medium and a cover slip is attached, the objective lens must be corrected for this additional thickness.
The correction factor is engraved on the objective immediately following the tube length (often 0.17 or 0.18 mm). A cover slip and mounting medium of that thickness must be used in order for the objective to maintain its optimal resolving power. If the value is listed as "-," the objective may be used with or without a cover slip.
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